The long term goals of this program project on Hearing Development are to understand the normal ontogeny of human hearing and the biological mechanisms underlying the occurrence of hearing disorders during development. the first steps toward achieving these goals are a more thorough understanding of the time course of hearing development in normal human infants, an understanding of structure-function relationships underlying auditory system ontogeny, and animal studies investigating the cellular mechanisms responsible for normal and abnormal development. toward these ends we have proposed seven interrelated research projects, all of which focus on development of hearing and the influence of experience on the auditory system. six of the projects investigate hearing, perceptual development, and language development in human infants, while one is concerned with the cellular mechanisms regulating development of central nervous system auditory neurons. A variety of techniques will be used to derive converging information on auditory development in human infants. For example, behavioral studies (Werner), physiological experiments (Folsom, Werner) and acoustical measurements of external, middle, and inner ear (Keefe, Folsom, Burns) will all address developmental changes in sensitivity, frequency selectivity, and temporal coding. Each of these studies provides important information for understanding the development of language processing and production, which will be directly investigated by Kuhl and Stoel-Gammon, who will also investigate cross-language comparisons to learn about the roles of experience in language production and perception. In addition the application of physiological, behavioral, and acoustical methods to hearing disabled children with Down's syndrome will be assessed. At a more cellular level we will continue investigating interactions between peripheral dysfunction and central nervous system development using morphological, immunohistochemical, and biochemical methods (Rubel). This entire research program is built on the philosophy that a number of well-equipped laboratories headed by senior investigators in close communication can use complimentary approaches and share resources to significantly advance our understanding of hearing development.